The IEEE 802.11 standard provides a set of Media Access Control (MAC) and Physical layer (PHY) specifications for implementing WLAN computer communication in the 2.4 gigahertz (GHz), 3.6 GHz, five (5) GHz, and sixty (60) GHz frequency bands. This standard is created and maintained by the IEEE Standards Committee IEEE 802. The base version of this standard was released in 1997 and has had subsequent amendments. This standard and its amendments provide the basis for wireless network products using the Wi-Fi bands.
A wireless local-area network (WLAN) links two or more devices using a wireless distribution method, and usually provides a connection through an access point (AP) to the wider Internet. This provides users the ability to move within a local coverage area and still be connected to the WLAN. All devices that may connect in a WLAN are referred to as a wireless station. A wireless station falls into one of two categories: an access point (AP) and a wireless client. An access point, which is typically a router, is a base station for a WLAN.
An access point transmits and receives signals at radio frequencies for wireless clients. A wireless client may be a mobile device such as a laptop, a personal digital assistant (PDA), an IP-phone, a smartphone or the like, or it may be a fixed device such as a desktop computer, a workstation or the like that is equipped with a wireless network interface. The IEEE 802.11 standard provides two basic modes of operation: an ad hoc mode and an infrastructure mode. In the ad hoc mode, a wireless client communicates directly peer-to-peer. In the infrastructure mode, a wireless client communicates through an access point that serves as a bridge to another network such as the Internet, a Local-Area Network (LAN) or the like.
A Wi-Fi system based on the IEEE 802.11 standard has many aspects in common with cellular systems. However, one difference is associated with the MAC protocol, which for a cellular system is typically scheduled, but for a Wi-Fi system is contention-based. This means that a receiving station does not know in advance from which transmitting station it will receive data as well as the format of the data.
The basic IEEE 802.11 MAC, the so-called Distributed Coordination Function (DCF), uses a Carrier-Sense Multiple Access with Collision Avoidance (CSMA/CA)-based MAC. The same protocol is applied by all stations including access points, i.e., in both downlink and uplink transmissions. This standard also supports a Point Coordination Function (PCF) mode, in which access points have more control over the medium usage. Supporting the PCF mode is however optional, and rarely implemented.
FIG. 1 illustrates a distributed coordination function (DCF) 100. In FIG. 1, a station using a DCF mode, User A, and wishing to transmit a frame of data must first sense a medium. If the medium is sensed to be idle for a certain minimum time, i.e. during a so-called Distributed Inter-Frame Space (DIFS), the data frame is transmitted. The DIFS is fifty microseconds (50 μs) in the release IEEE 802.11b standard.
In FIG. 1, if the medium is busy, as it is for user C, the station first waits until the medium is sensed to be idle, as represented by the reference “defer.” When this occurs, the station additionally defers the transmission during a DIFS. Since an immediate transmission may lead to collisions if more than one station is waiting until the medium is sensed idle, the station sets a back-off timer to a random delay and transmits only when this back-off timer has expired, instead of transmitting immediately when the medium is sensed idle. The back-off timer is only activated when the medium is sensed idle. Whenever the medium is sensed busy, a deferral state is entered in which the back-off timer is not activated. When the back-off timer expires, the data frame is transmitted.
If the data frame is successfully received by a station, the receiving station responds with an acknowledgement to the transmitting station. The acknowledgement is sent a Short Inter-Frame Space (SIFS) after the data frame is received. The SIFS is ten microseconds (10 μs) in the release IEEE 802.11b standard. Since SIFS is shorter than DIFS, no other station will access the medium during this time. If no acknowledgement is received by the transmitting station, the transmitting station generates a new back-off timer value, and retransmits the frame when the new back-off timer has expired.
The reason for not receiving any acknowledgement may be because either the transmitted data frame is lost, resulting in no acknowledgement being returned, or because the acknowledgement itself is lost. Even if the data frame is successfully acknowledged, the transmitting station must generate a back-off timer value and wait for it to expire before transmitting the next frame. This is to enable other stations to grab the channel.
To avoid congestion when collisions occur, back-off timer values are drawn from distributions with larger and larger expected values for every retransmission attempt. For the nth transmission attempt, the back-off timer value is drawn from the uniform distribution ∪[0, min((CWmin)2n-1−1, CWmax]. CWmin and CWmax are constants with values depending on the physical layer. For the release IEEE 802.11b standard, CWmin is thirty-one (31) and CWmax is one thousand and twenty-three (1023). The back-off timer value is measured in units of slot times, which for the release IEEE 802.11b standard are twenty microseconds (20 μs).
In the Enhanced DCF mode, defined in the release IEEE 802.11e standard, service prioritization is introduced. This is done by using back-off and deferral parameters that depend on service type.
Since frames are transmitted after a DIFS when the medium is free, the minimum delay is equal to the transmission time plus a DIFS, which for release IEEE 802.11b is about one millisecond (1 ms) for a fifteen hundred (1500) byte data frame. The almost immediate acknowledgement, with a transmission time of around one-tenth of a millisecond (0.1 ms), means that the Round-Trip Time (RTT) on layer 2 is on the order of one millisecond (1 ms).
Because of the back-off and deferral times between transmissions, the medium is not fully used even at high traffic loads. The maximum link utilization reached depends on the frame size, and varies from fifty percent (50%) for voice, to seventy percent (70%) to eighty percent (80%) for data.
Licensed-Assisted Access, Long-Term Evolution (LAA-LTE) is one of the main work items for the LTE Release 13 standard under the umbrella of 3GPP. It proposes to use unlicensed bands (e.g. 2.4 GHz and 5.1 GHz) for LTE or LTE-like transmission in coexistence with other wireless standards (like WLAN IEEE 802.11 and Bluetooth). The primary channel of LTE in the licensed band serves as the main connection while secondary carrier(s) are set up in unlicensed bands to boost the throughput to the user. Most of today's available unlicensed bands are used by WLAN and the collisions between both systems may significantly reduce performance on both.
Especially in the five gigahertz (5 GHz) unlicensed spectrum bands, there is a multitude of channels available which could be used for LTE transmission if a suitable coexistence protocol is designed. Such a protocol must not only cope with coexistence with WiFi, but also handle coexistence between LTE networks from different operators all trying to use the unlicensed spectrum.
WiFi is asynchronous, as opposed to LTE, since WiFi transmissions may happen at any time if the channel is free. Moreover, the transmissions have a variable size which is signaled in the preamble of the WiFi data frame. However, LTE transmissions follow a rigid frame structure and are required to be aligned to the Transmission-Time Interval (TTI) of one millisecond (1 ms). Both the transmitter and the receiver of the wireless device are aligned to the TTI and the duration of transmission is in quanta of this TTI.
An important feature in LAA-LTE, not present in WiFi, is the possibility of the licensed carrier to be used as a control channel for grants, acknowledgments and the like, while off-loading the data to the secondary channel, if available. This may allow more users on the licensed spectrum where they may benefit from the multi-user capabilities of the LTE system, while the off-loaded data is transmitted using “best effort” without scheduling but with the added benefit of utilizing the LTE carrier for robust control signaling.
Listen-Before-Talk (LBT) is a protocol where the desired channel on the wireless medium is first sensed for any potentially interfering transmissions before a transmission begins. If the medium is found free, then the transmitter may start using it. Together with a back-off mechanism, an LBT protocol potentially avoids collisions. See for example the Load-Based Equipment (LBE) protocol described in ETSI EN 301 893 V1.7.2 (2014 July), entitled “Broadband Radio Access Networks (BRAN); 5 GHz high performance RLAN; Harmonized EN covering the essential requirements of article 3.2 of the R&TTE Directive.” The IEEE 802.11 standard uses another such method called CSMA/CA (Channel-Sensing Multiple Access/Collision Avoidance). See IEEE Std 802.11-2012 (Revision of IEEE Std 802.11-2007) (IEEE 29 Mar. 2012), entitled “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications.” This along with DCF, as described in Part 11 of IEEE Std 802.11-2012, forms a way of avoiding collisions.
The LBT protocols usually consist of a number of steps that include: Listen to medium—measure the received signal with procedures such as Clear-Channel Assessment (CCA); Decision of medium busy/free—based on energy detection and/or decoding of signal; and Start transmission—if medium is free or after a defined back-off period, the system starts transmission.
Currently in LTE, the radio is, at least conceptually, a simple device. It transforms the baseband signal it receives from Layer 1 into a radio transmission in a predefined frequency band. There is also a delay from the time that the Layer 2-based scheduler has assembled the signal until it reaches the radio and, subsequently, appears on the medium. This is typically on the order of one millisecond (1 ms) to two milliseconds (2 ms) in LTE. There is no channel sensing in LTE, but rather it is the responsibility of the scheduler to make sure there are no collisions on the channel. The strict synchronicity of all transmissions to TTIs/subframes makes this possible.
One of the problems with existing solutions is that there may be no mechanism in LTE networks for the radio to perform channel sensing in a fast way adapted to operate in unlicensed bands with existing technologies such as WiFi. In other words, current mechanisms for LTE systems may not transition from receiving or sensing to transmitting fast enough (<15 μs) to grab the channel in the WiFi bands. Hence, an LBT-functionality suitable for the WiFi band is difficult to achieve. Accordingly, there is a need for techniques to improve performing a listen-before-talk (LBT) protocol in a wireless device. Furthermore, other desirable features and characteristics of the present disclosure may become apparent from the subsequent detailed description and claims, taken in conjunction with the accompanying figures and the foregoing technical field and background.
The Background section of this document is provided to place embodiments of the present disclosure in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section.